In recent years, in view of reducing the environmental burden, there have been demanded environmentally friendly clean liquid fuels which are lower in sulfur content and aromatic hydrocarbon content. With the above view taken into account, as a technique capable of producing a fuel oil base stock which is free of sulfur and aromatic hydrocarbon content but rich in aliphatic hydrocarbon, in particular, kerosene and gas oil base stocks, there has been studied a process for utilizing the Fischer-Tropsch synthesis reaction (hereinafter, referred to as the “FT synthesis reaction”) using carbon monoxide gas (CO) and hydrogen gas (H2) as a feedstock gas. This method is that in which a natural gas is reformed to produce a synthesis gas (a mixed gas containing CO and H2 as main components), the synthesis gas is subjected to the FT synthesis reaction, thereby synthesizing hydrocarbons with a wide carbon number distribution, and the obtained hydrocarbons are hydrogenated and fractionally distilled to produce liquid fuel base stocks. This is referred to as the GTL (Gas To Liquids) technique (refer to Patent Document 1, for example).
As a process for producing hydrocarbons by the FT synthesis reaction, there is also a known process using a slurry bubble column reactor in which a synthesis gas is blown into a slurry prepared by suspending solid catalyst particles within liquid hydrocarbons (hereinafter, from time to time simply referred to as “slurry”) to conduct the FT synthesis reaction (refer to Patent Document 2, for example).
In the process using the slurry bubble column reactor, a gas phase portion at an upper part of the slurry inside the reactor is formed, and a line connected to an upper part of the reactor discharges through a synthesis gas which remains unreacted during passage through the slurry (unreacted synthesis gas) and light hydrocarbons which are produced by the FT synthesis reaction and kept in a gaseous state under conditions inside the reactor.
In the above-described slurry bubble column reactor, normally, in order to separate and recover the light hydrocarbons discharged through the line connected to the upper part of the reactor, the line is connected to a gas-liquid separator, gas components discharged from the upper part of the reactor are cooled by a cooler of the gas-liquid separator, and condensed light liquid hydrocarbons are separated from the gas components by a gas-liquid separation vessel. Then, the separated gas components containing an unreacted synthesis gas are recycled into the reactor, and separated liquid components (light liquid hydrocarbons) are supplied to a subsequent step of distillation together with heavy hydrocarbons to be described later. Here, the heavy hydrocarbons produced by the FT synthesis reaction are fundamentally drawn out as liquids from the slurry bed of the reactor, with a slight vapor pressure kept under conditions inside the reactor. Thus, the heavy hydrocarbons partially exist as a gas in a gas phase portion and discharged as a portion of the gas components discharged through the line. Further, liquid heavy hydrocarbons may be entrained with a gas in the form of droplets and contained in the discharged components.